GPS (Global Positioning System) is an earth-satellite-based electronic system for enabling GPS receivers in ships, aircraft, land vehicles and land stations to determine their geographic and spatial position such as in latitude, longitude, and altitude. Discussion of GPS herein is without limitation to other analogous electronic systems as well as applicable receiver circuits in a variety of telecommunication systems.
It would be desirable to even more accurately, reliably, rapidly, conveniently and economically search for, acquire, and track received signals and maintain accurate time, position, velocity, and/or acceleration estimation in a communication device having a satellite positioning receiver (SPR) or other receiver and its clock source.
Current GPS (Global Positioning System) receivers and other positioning and communications receivers are expected to operate in high dynamic range (e.g., a range of 38 db or so) of receive powers from different satellites and other transmitting sources. The cross correlation protection provided by the 1 ms PN (pseudorandom noise) codes used by GPS satellites, for instance, is not sufficient for such a large dynamic range, which is problematic. The cross correlation problem occurs mainly when the Dopplers are close by or same as each other or their Doppler difference is close to a one kilohertz (1 Khz) or a multiple of 1 KHz. The 1 KHz or multiple is involved because of the periodicity of the PN-code, e.g., 1 ms in GPS.
Some positioning receivers such as GPS receivers need to operate in indoor environments and/or for urban canyon environments and otherwise where the dynamic range of receive satellite signal powers is high, typically 25-35 db or more. Cross correlation interference is a problem especially in GPS receivers because the PN-codes are not designed to operate in such wide power ranges. For another example, the open-sky power level can be as high as −125 dBm whereas current GPS receivers can target to acquire and track signals with power level as low as −163 dBm or smaller. The GPS system was not designed for such a large dynamic range of nearly 38 db. The basic cross correlation protection offered by the PN codes of various satellites is only about 20 db. Cross correlation issues arise with zero and non-zero Doppler-difference multiples of 1 KHz across satellites.
Cross correlation mitigation techniques hitherto have been undesirably conservative or pessimistic in nature and tend to reject valid lower power satellite vehicle (SV) signals as cross correlation. One approach, called Approach 1 here, involves a power difference determination. If a low power peak is detected at a Doppler difference of one kilohertz or multiple away from already detected higher power SV2 peak and if a difference of SV signal powers C/No is 18 dB or more, then the low power peak is declared or regarded as a cross correlation peak and not used. Such threshold of 18 db is based on or derived from the worst-case cross correlation coefficient between two PN codes. This detection approach is undesirably conservative or pessimistic in nature because the low power measurement could be a genuine or uncorrupted valid lower power peak of an actual satellite vehicle, designated SV1 here.
Limitations of such approach are first, being undesirably conservative in nature and regarding even true lower power peaks as suspect and rejecting them as if they were undesirable cross correlation peaks. This results in a starvation of positioning and other navigational measurements in difficult satellite visibility conditions. It is believed that one cannot make such conservative approach more aggressive without undesirably impacting the probability of false detection that such approach by nature also involves.
New GNSS (Global Navigation Satellite Systems) standards are being designed to have better cross correlation rejection properties. The Russian system called Glonass uses FDMA (Frequency Division Multiple Access) technique wherein each satellite vehicle SVx uses a different carrier frequency and provides cross correlation rejection of around 45 db or higher. The European system called Galileo uses a longer pseudo-noise PN code (4 ms, 4092 long, four times longer than GPS), thereby having less issue with cross correlation compared to the cross correlation problem that GPS poses. This disclosure is applicable to Galileo as well, since even Galileo will have cross correlation problems, even though it is better than GPS. However, GPS is a widely used GNSS and the need for improved GPS receivers is substantial both in terms of the cross correlation performance issue itself as well as the large market for GPS receivers. Moreover, the cross correlation problem can be expected in any GNSS whether GPS or otherwise in which the dynamic range of received satellite signal powers exceeds in magnitude the cross correlation protection offered by the code. Accordingly, as GNSS receivers with wide and wider dynamic ranges become increasingly pervasive in the market, this problem can increasingly confront users of GPS and any other applicable GNSS. In addition, this problem can confront users of any spread spectrum system such as CDMA (code division multiple access) processing received signals of wide enough dynamic range relative to cross correlation protection.